MEMS (Micro Electro-Mechanical Systems) applications, for example, often require signal amplifiers having a small area requirement, a low input capacitance, a high input impedance, a small current requirement and low noise. A rail-to-rail AB output stage, current feedback and input signal levels up to the supply voltage, for example an input signal based on a negative or positive supply, are also desirable.
In a conventional amplifier, a sufficient gain can be provided using a transistor in a source-follower configuration, as an input buffer, and a downstream differential amplifier. In this case, the differential amplifier comprises a feedback voltage divider on the output side.
However, the problem with such an amplifier is that there is a relatively high noise level.
Alternatively, an input signal based on a reference between the supply voltages could also be used. However, the power consumption, the complexity of the circuit and the noise would be greater as a result. In the case of a fully differential stage based on the supply voltage Vss, additional common mode potential regulation would be required. This would likewise result in undesirable circuit complexity, noise and power consumption. Alternatively, an input transistor of an amplifier could also be connected into the source circuit. However, the gate-source capacitance would be present at the input node as a result. This would result in undesirable loading of the signal source. In addition, stray capacitances at the input could not be shielded.